Currently, nonaqueous-electrolyte secondary batteries in which charge-discharge is, for example, performed by allowing lithium ions to move between a positive electrode and a negative electrode using a nonaqueous electrolyte are widely used as secondary batteries with high energy density.
In such a nonaqueous-electrolyte secondary battery, a lithium-transition metal composite oxide having a layered structure such as lithium nickelate (LiNiO2) and lithium cobaltate (LiCoO2) is commonly used as a positive electrode, and a carbon material, lithium metal, a lithium alloy or the like capable of occluding and releasing lithium ions is used as a negative electrode (for example, see Patent Document 1). A lithium ion capacitor having a negative electrode in which lithium ions are added to a carbon material capable of occluding lithium ions has also been developed.
However, the amount of lithium resource is relatively limited, and lithium is expensive. Further, reserves are unevenly distributed, and many are found in South America. Consequently, Japan needs to import lithium entirely from abroad. Accordingly, sodium-ion secondary batteries intended for use in place of lithium-ion secondary batteries are currently under development for stable supply of low cost batteries. However, a carbon material which can be used for them is limited to hard carbon (for example, see Patent Document 2).
Recently, researchers have begun to explore the possibility of nonaqueous-electrolyte secondary batteries in which potassium ions are used instead of lithium ions and sodium ions. Potassium, which is abundantly contained in both sea water and earth crust, is a stable resource, allowing for low cost products. Proposed is a potassium-ion secondary battery having a current collector as a negative electrode, the current collector being prepared by applying a slurry to a copper foil, the slurry being prepared by mixing graphite as a negative-electrode active material with poly(vinylidene fluoride) (PVdF) as a binder in a mass ratio of 95:5 (for example, see Patent Document 3).
Potassium is known not to make an alloy with aluminum or copper at ordinary temperature (for example, see Non Patent Documents 1 and 2). Further, computational chemistry studies have shown that potassium may have a fast diffusion rate in graphite (for example, see Non Patent Document 3).
Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2003-151549
Patent Document 2: Japanese Unexamined Patent Application, Publication No. 2013-229319
Patent Document 3: Japanese Unexamined Patent Application, Publication No. 2006-216511
Non Patent Document 1: Pelton, A. D., “The Cu—K (Copper-Potassium) system.”, Bulletin of Alloy Phase Diagrams, 1986, 7 (3), 231-231.
Non Patent Document 2: Du, Y., and other three persons, “Thermodynamic modeling of the Al—K system.”, Journal of Mining and Metallurgy, Section B: Metallurgy, 2009, 45 (1), 89-93.
Non Patent Document 3: Wang, Z., and other three persons, “Diffusion of alkali metals in the first stage graphite intercalation compounds by vdW-DFT calculations.”, RSC Advances, 2015, 5 (21), 15985-15992.